How can DNA be so long when the cells are so small?
(Lansing State Journal, November 17,1993)
Consider a human cell to be a very small square box. On average it will have sides with lengths of about 10 microns. (A micron is 10-6 meters). This is reasonable, and can be verified by directly measuring cells under a microscope. The volume of such a cube would be 10-15 cubic meters. Now, DNA can be modeled as a cylinder with a radius of 1 nanometer (10-9 meters). We know that this is more or less correct, because studies of DNA using X-ray spectroscopy show that DNA forms long thin helixes of measurable dimensions. If the length of the DNA is really .99 meters, then its volume would be 3.1x 10-18 cubic meters. That means that DNA takes up less than 1 percent of the cell's volume!
In fact, a person's DNA is split up into separate pieces called chromosomes. And these chromosomes are kept in a special compartment in the cell called the nucleus. The nucleus itself may take up to 10 percent of the cell's volume, because it contains a variety of proteins and RNA as well as DNA.